charging the 3800 mah extended battery

okay I noticed a big difference when charging the extended battery when the phone is on and turned off. When the phone is on and i charge the battery the battery charges really fast, however, i only get like 18 hours moderate use on 1 charge. Now when the phone is turned off and charging the battery it takes like 8 hours to charge, however, I get an incredible 3 days moderate use on 1 charge. Has anybody noticed how differently the extended battery charges phone turned on versus turned off....has anybody else experienced this before.....?????

I need feedback on this please